Pulsed-neutron nuclear logging tools operate by releasing high energy neutrons, on the order of 14 Million electron Volts (MeV), into the borehole and formation. The neutrons create gamma particles or gamma rays (hereafter just gammas) by one of several of interactions with the surrounding atoms: 1) the neutrons inelastically collide with atomic nuclei and thereby create gammas (known as inelastic gammas); and 2) when a neutron loses energy (possibly through the inelastic collisions or by other means) the neutron is captured by the nucleus of a nearby atom and in the process a gamma is emitted (known as a capture gamma). The atoms with which the neutrons collide and/or the neutrons are captured can be determined by the energy (or frequency) of the gammas produced.
When using a pulsed-neutron nuclear logging tool as a bulk density measurement device, inelastic gammas carry most of the information as to the formation bulk density. When using a pulsed-neutron nuclear logging tool to determine formation lithology or the macroscopic capture cross-section, it is often the capture gammas that carry most of the information. However, elements of the various components that are in, or which form, the borehole (e.g., drilling fluid, casing, cement, production tubing, or the logging tool itself) also interact with neutrons, and thus it is difficult in the related art to distinguish gammas originating within the borehole from those originating within the formation.